Relationship of masked obesity to self-reported lifestyle habits, ideal body image, and anthropometric measures in Japanese university students: A cross-sectional study

Introduction Masked obesity (MO) is defined as a normal body mass index (BMI) with a high body fat percentage (%BF), and is associated with the onset of lifestyle-related diseases. However, little is known about the current status of MO. Therefore, we investigated the relationship of MO to physical characteristics and lifestyle habits among Japanese university students. Methods Between 2011 and 2019, we conducted a survey of 10,168 males and 4,954 females with BMI within the normal range (18.5 ≤ BMI < 25 kg/m2). MO was defined as %BF ≥ 20% in males and %BF ≥ 30% in females. Students completed a questionnaire containing items about lifestyle habits. Systolic and diastolic blood pressures were measured, and hypertension was defined as systolic above 140 mmHg and/or diastolic blood pressure above 90 mmHg. A multivariate logistic regression analysis was performed to evaluate the relationships as follows: the relationship of masked obesity to self-reported lifestyle habits, ideal body image, and anthropometric measures; the relationship between hypertension and body indices. Results The proportion of students with MO in 2019 was 13.4% in males and 25.8% in females, and the proportion of females increased over time. MO was associated with desire for weight loss (odds ratio, 95% confidence interval: 1.76, 1.53–2.02), intake of five macronutrients (0.79, 0.67–0.93), rice and wheat intakes (1.22, 1.01–1.47), sleep duration of < 7hr (0.85, 0.74–0.98), and exercise habit (0.71, 0.63–0.81) in males; and with balanced diet intake (0.79, 0.64–0.99) and exercise habit (0.65, 0.51–0.82) in females. There was a significant association of MO with hypertension in males (1.29, 1.09–1.53). Conclusion The percentage of female students with MO increased during the study period, and in males, MO may be a risk factor for hypertension. These results suggest that intervention for MO is needed in Japanese university students.


Introduction
Masked obesity (MO) is defined as a normal body mass index (BMI) with a high body fat percentage (%BF), and is associated with the onset of lifestyle-related diseases. However, little is known about the current status of MO. Therefore, we investigated the relationship of MO to physical characteristics and lifestyle habits among Japanese university students.

Methods
Between 2011 and 2019, we conducted a survey of 10,168 males and 4,954 females with BMI within the normal range (18.5 � BMI < 25 kg/m 2 ). MO was defined as %BF � 20% in males and %BF � 30% in females. Students completed a questionnaire containing items about lifestyle habits. Systolic and diastolic blood pressures were measured, and hypertension was defined as systolic above 140 mmHg and/or diastolic blood pressure above 90 mmHg. A multivariate logistic regression analysis was performed to evaluate the relationships as follows: the relationship of masked obesity to self-reported lifestyle habits, ideal body image, and anthropometric measures; the relationship between hypertension and body indices.

Results
The proportion of students with MO in 2019 was 13.4% in males and 25.8% in females, and the proportion of females increased over time. MO was associated with desire for weight loss (odds ratio, 95% confidence interval: 1.76, 1.53-2.02), intake of five macronutrients

Introduction
Masked obesity (MO) is defined as normal body mass index (18.5 � BMI < 25 kg/m 2 ) but with a high body fat percentage (%BF). Various definitions of MO have been proposed in previous studies [1][2][3][4][5][6]. Fujise et al. [2] defined MO as a normal BMI with %BF of � 20% in males and � 30% in females. In a study of 8,068 young females [1], the proportion of participants with BMI � 25 kg/m 2 and %BF � 30% was 7.4% of the total study population. Similarly, the proportion of young females with normal BMI and with %BF � 30% has been reported as 32.0% [4]. Oguri et al. [5] reported that total cholesterol and triglyceride levels were significantly higher in those with MO. MO could induce lifestyle-related diseases such as hypertension, and the relationship should be investigated. BMI is used to determine obesity; however, it is not a precise measure of body fat and thus may not accurately reflect the amount of body fat in an individual. New anthropometric indices that can estimate body fat distribution have recently been suggested for evaluation of obesity, including the body roundness index (BRI), body shape index (ABSI), body adiposity index (BAI), abdominal volume index (AVI), and conicity index (CI) [7,8]. As these anthropometric indices can be a useful predictor of MO, it is necessary to investigate the relationship between them and MO; however, there are limited data on these relationships among Japanese university students.
It has been reported that young Japanese females with MO tended to have no exercise habit, more dieting (restricting calorie intake) experience, and lower intakes of vegetables and beans [4]. Similarly, MO groups were reported to consume fewer vegetables per day and consumed mayonnaise and fried foods more frequently compared with groups of normal body type [9]. Furthermore, poor eating behaviors that are caused by a desire for weight loss may induce MO [2]. Females are known to have a stronger desire for weight loss than males, and numerous studies have investigated the desire for weight loss and weight loss experienced by female university students [10][11][12][13]. Additionally, most previous studies [2,4,9] have focused on MO among female students, and corresponding data on male students is lacking. Thus, there is a need to investigate the relationship of MO to lifestyle habits and the desire for weight loss among both male and female Japanese students.
University students are known to have more flexible daily schedules compared with high school students and adults, as well as more inappropriate eating habits [14] and sleeping habits [15]. Physical characteristics that are apparent in young adulthood are not temporary, and are likely to be associated with those in middle age [16]. Additionally, MO could induce lifestylerelated disease such as hypertension, and it is desirable to improve inappropriate lifestyle habits during the early stage of life. Therefore, this study aimed to investigate the relationship of MO to physical characteristics and lifestyle, and evaluate its impact on the blood pressure of Japanese university students.

Participants and survey period
The survey population consisted of students who were enrolled at Kindai University between 2011 and 2019. The number of students in the first grade is approximately 5,000 males and 2,500 females every year. Subjects for whom data on sex, age, lifestyle habits, blood pressure, and anthropometric measures were missing or unclear were excluded. Finally, 10,168 males and 4,954 females with BMI within the normal range (18.5 � BMI < 25 kg/m 2 ) were eligible for participation in the study. MO was defined as %BF � 20% in males and � 30% in females [2]. Although Fujise et al. [2] defined the normal range of BMI as 19.8 to 24.2 kg/m 2 , our study used the current standards of the Japan Society for the Study of Obesity (JASSO) [17]. In the total eligible population, MO was identified in 1,868 male and 915 female students (MO group). Those without MO were included in the normal group.
Survey content and methodology S1 Table lists questionnaire items related to age; sex; the desire for weight loss; and eating, sleeping, and exercise habits. The survey was conducted using self-administered questionnaires that were distributed, completed, and then collected during a university lecture. In terms of ideal body weight, students who answered that they "want to lose weight" were classified as the "decreasing group," those who answered that they "want to maintain the same weight" were defined as the "maintenance group," and those who answered that they "want to gain weight" were defined as the "increasing group." Regarding snack intake, the respondents were asked to choose between "Daily," "Occasionally," "I used to snack," or "I do not snack." Those who answered "I do not snack" or "I used to snack" were classified as the "no-snacking group," and those who answered with another option were classified as the "snacking group." The respondents were asked to self-evaluate their intake of nutrients on a 10-point scale (1: dissatisfied to 10: satisfied). The items presented were five macronutrients, rice/wheat, meat, fat, vegetables, fruit, and the levels of their balance. Each item was dichotomized as the "lowvalue group" (� 5 points) and "high-value group" (� 6 points). Sleep duration was categorized as < 7 hours, 7 �, < 8 hours, or � 8 hours; and sleep onset time as � 15 minutes and < 15 minutes. The median sleep onset time was 15 minutes in male and female students in this study. In addition, exercise habit was defined based on the National Health and Nutrition Survey [18] as "exercise frequency" of at least twice a week, and as "exercise duration" of at least 30 minutes per exercise session.

Blood pressure, anthropometric measures, and body indices
Systolic and diastolic blood pressures (SBP and DBP, respectively) were measured at the level of the heart after 5 minutes rest in the sitting position, using an automatic electronic sphygmomanometer (HEM-1010; Omron Dalian Co., Ltd., Kyoto, Japan). Hypertension was defined according to The Japanese Society of Hypertension (JSH) guidelines [19] as SBP > 140 mmHg and/or DBP > 90 mmHg. At the same time as blood pressure measurements were obtained, anthropometric parameters of height, weight, waist circumference (WC), and hip circumference (HC) were measured to the nearest 0.1 cm or 0.1 kg in the standing position, with the participants wearing minimal clothing. The %BF was measured using a dual-frequency body composition meter (DC-320 or DC-430A; Tanita Co., Tokyo, Japan). WC was measured at the level of the umbilicus, and HC was measured at the widest part around the buttocks. BMI, WC to HC ratio (WHR), and WC to height ratio (WHtR) were calculated using the following formulae. The body indices of ABSI [20], AVI [21], BAI [22], BRI [23], and CI [24] were calculated using the following formulae.

Statistical analysis
The Cochran-Armitage trend test was used to evaluate the proportion of MO in males and females over time. The Mann-Whitney U test was performed to compare anthropometric indices between the MO and normal groups. Receiver-operating characteristic (ROC) curve analysis was conducted to assess the predictive performance of the anthropometric indices for MO by calculating the area under the curve (AUC). The sensitivity and specificity of each anthropometric index were also determined using ROC analysis. The maximum value of Youden's index (sensitivity + specificity − 1) was considered the optimal cut-off point for each anthropometric index, and the students were classified into high and low groups based on each cut-off point. Furthermore, a multivariate logistic regression analysis was performed to consider confounding factors. The objective variable was set to MO, and the explanatory variables were set to ideal body weight (decreasing or increasing vs. maintenance), eating habits, sleep duration (< 7 hours or � 8 hours vs. 7 �, < 8 hours), sleep onset time, and exercise habits. Additionally, a multivariate logistic regression analysis was performed to evaluate the relationship between hypertension and body indices. The objective variable was set to hypertension, and the explanatory variables were set to BMI, WC, and MO in males, and WC in females. A forward and backward stepwise selection with a significance level of 0.05 was performed to evaluate the effectiveness of the explanatory variables. Windows JMP Pro ver. 15.00 (SAS Institute Inc., Cary, NC, USA) was used for statistical processing and significance tests, with a significance level of less than 5%.

Ethical considerations
This study protocol was approved by the Research Ethics Committee of Kindai University (approved ID: 08-001, 12-033, and 15-072) and all participants provided written informed consent. The participants were informed that their data would only be used for research on antiaging or prevention of lifestyle-related diseases, that participation in this survey was voluntary, and that participants' privacy would be guaranteed. There were no participants younger than 18 years of age.  Table 1 shows the results of comparison of anthropometric indices in the MO and normal groups. Table 2  respectively, in females, and significant differences were found for BMI between the MO and normal students (p < 0.001) despite the equal BMI ranges. There was no significant difference in age or height between the MO and normal groups. All other physical measurements were significantly higher in the MO group than in the normal group (p < 0.01   Tables 3 and 4 show the relationship of MO to ideal body weight and lifestyle habits. The percentages of male students in the MO and normal groups were 69.2% and 32.3% in the decreasing group, 23.9% and 47.3% in the maintenance group, and 6.9% and 20.4% in the increasing group, respectively. Multivariate logistic regression analysis showed that MO in male students was significantly associated with BMI (OR: 2.49, 95% CI: 2.37-2.61, p < 0.001) and the desire for weight loss (OR: 1.76, 95% CI: 1.53-2.02, p < 0.001), five macronutrient intakes (OR: 0.79, 95% CI: 0.67-0.93, p = 0.004), rice and wheat intakes (OR: 1.22, 95% CI: 1.01-1.47, p = 0.042), sleep duration of < 7hr (OR: 0.85, 95% CI: 0.74-0.98, p = 0.022), and exercise habit (OR: 0.71, 95% CI: 0.63-0.81, p < 0.001). In contrast, the percentage of female students who had the desire for weight loss was 98.5% in the MO group and 89.6% in the normal group, indicating that the majority of females had the desire for weight loss. Multivariate logistic regression analysis showed that in female students, MO was significantly associated with BMI (OR: 3.59, 95% CI: 3.31-3.90, p < 0.001), balanced diet intake (OR: 0.79, 95% CI: 0.64-0.99, p = 0.044) and exercise habit (OR: 0.65, 95% CI: 0.51-0.82, p < 0.001). Table 5 shows the relationship of hypertension to BMI, WC, HC, and MO. In male students, a significant positive association was observed between hypertension and BMI (p < 0.001), WC (p = 0.005), and MO (p = 0.004), with OR (95% CI) of 1.14 (1.08-1.20), 1.02 (1.01-1.04), and 1.29 (1.09-1.53), respectively. In female students, a significant positive association was observed between hypertension and WC (p < 0.001), with OR (95% CI) of 1.06

Discussion
MO is defined as normal BMI but with high %BF. Suzuki et al. [25] defined MO as weight of < 120% of the JASSO standard BMI, with %BF of � 25% in males and � 30% in females. Fujise et al. [2] defined MO as a normal BMI based on JASSO and a %BF of � 20% in males and � 30% in females. Based on that study, we defined MO as 18.5 � BMI < 25 kg/m 2 and % BF of � 20% in males, and %BF of � 30% in females. In the present study, the percentages of male and female students with MO were 18.4% and 18.5%, respectively. In addition, the percentage of male students with MO decreased, whereas that of female students increased over time between 2011 and 2019. The reason for the decrease in the percentage of male students with MO over time remains unclear; however, there is concern that MO could continue to increase in females in the future.
In the present study, the mean BMI values of male and female students with MO were 23.0 kg/m 2 and 22.8 kg/m 2 , respectively, which is significantly higher than those of the normal groups. Additionally, mean WHR values of the MO and normal groups were 0.86 and 0.83 for male students and 0.80 and 0.77 for female students, respectively, and the mean was significantly higher in the MO groups than in the normal groups. WHR is used as an indicator of obesity, with a cutoff value of � 0.95-1.0 in males and � 0.80-0.85 in females [26]. In the present study, the cutoff values of BMI and WHR for predicting MO were 22.0 kg/m 2 and 0.84 in males and 21.4 kg/m 2 and 0.78 in females, respectively, which are lower than those for predicting obesity reported in previous studies [17,26]. Recently, new anthropometric indices such as ABSI, AVI, BAI, BRI, and CI, which can estimate body fat distribution, have been used to evaluate obesity [7,8]. However, little is known about the relationship between MO and anthropometric indices among Japanese university students. We compared the AVI and BRI values, which had high predictive ability in our study, with those reported in previous studies for predicting metabolic syndrome [8,27,28]. The cutoff values of AVI in previous studies were 14.3-16.2 in males and 12.8-13.0 in females, whereas those of the present study were 11.7 in males and 10.4 in females. In addition, the cutoff value of BRI in previous studies was 3.47-3.94 in males and 3.43-3.77 in females, whereas that of our study was 2.42 in males and 2.43 in females. The cutoff values of AVI and BRI were lower in the present study than those in previous studies, which suggests that new cutoff values should be set for determining MO using anthropometric indices, and that the criteria should include sex and age. The percentage of the present female students with the desire for weight loss was 98.5% in the MO group and 89.6% in the normal group. Japanese females have a strong desire for weight loss, as reported in a study of ideal body image that found more than 90% of female university students "want to be thin" [29]. More than 70% of female university students have experienced dieting behaviors [29]. It has been suggested that these desires and behaviors are risk factors for MO [2,4]. Thus, intervention for appropriate assessment of body shape is needed in Japanese university students. Meanwhile, the percentage of the present male students with the desire for weight loss was higher in the MO group than in the normal group. Kagawa et al. [30] suggested that Japanese males are more likely to overestimate their own weight than are western males. Although there are limited data on a desire for weight loss among males, such a desire may induce inappropriate dieting behaviors that are risk factors for MO.
The MO groups in the present study tended to have low intake of the five macronutrients (in males) and low intake of a balanced diet (in females). A previous study found that young Japanese females with MO had low intake of vegetables and beans [4], in agreement with the present results. Further research is needed to identify the exact amounts of intake for each nutrient, which was not possible in the present study as the survey method was based on a selfrating of lifestyle habits. According to the National Health and Nutrition Survey [18], the frequency of intake of a balanced diet is lower among Japanese aged 20-29 years than in other generations. It is also a concern that the easy availability of food at fast food restaurants, convenience stores, and vending machines has led to disordered eating habits among young people [31]. Intake of a well-balanced diet is important, with each nutrient consumed without excess or deficiency.
The percentage of male students with sleep duration of < 7 hours was lower in the MO group than in the normal group. The percentage of those who average 7-8 hours of sleep per day is highest among Japanese aged 20-29 years, but the percentage of those who feel sleepy during the daytime is high [18]. This is due to the pre-bedtime use of cell phones and video games, which can interfere with sleep quality. Furthermore, it has been reported that the prevalence of hypertension and metabolic syndrome, which are risk factors for cardiovascular disease, was higher in those with sleep duration of < 7 or � 8 hours compared with those with sleep duration of 7-8 hours, and the relationship between sleep duration and prevalence showed a U shape [32,33]. Although the percentage of MO was lower in those with sleep duration of < 7 hours, the results of previous studies suggest that it is important to ensure 7-8 hours of sleep. The present study found that compared with the normal group, fewer male and female students in the MO groups were in the habit of exercising. Another study has reported that young Japanese females with MO had no exercise habits [4]. MO can be induced by a decrease in muscle mass and bone mass caused by a lack of exercise [2]. Meanwhile, data from the National Health and Nutrition Survey [18] reveal that the percentage of Japanese aged 20-29 years with exercise habits is 28.4% in males and 12.9% in females, with no significant fluctuation over time in males but a significant decrease in females over the past decade. Regarding the desire to improve their exercise habits, data from the National Health and Nutrition Survey [18] reveal the percentage of respondents who answered "interested but have no orientation to improve" was high among males and females aged 20-29 years, and was particularly high among those who had no exercise habits. Therefore, how exercise habits can be established is an issue to be addressed in the future.
In the present study, hypertension was significantly associated with BMI, WC, and MO in male students, and with WC in female students. Uchiyama et al. [34] found that hypertension in Japanese university students was associated with higher BMI, and the prevalence increased as BMI increased. In a previous study that defined MO as weight of < 120% of the JASSO standard BMI, with %BF of � 25% in males and � 30% in females, SBP and DBP values in participants who underwent health screening were higher in the MO group than the normal group [25]. In the present study, however, we did not observe a significant relationship between hypertension and MO in female students, possibly because of the small percentage of female students with hypertension. These findings suggest that MO may be a risk factor for hypertension as well as obesity. We additionally observed a significant positive relationship between hypertension and WC in both male and female students in our study. The diagnostic criteria for metabolic syndrome are established as WC of � 85 cm in males and � 90 cm in females [35]. However, our results showed that mean WC was within these limits even in the MO groups. In a previous study of university students [36], the mean value of WC was 73.3 cm in males and 67.8 cm in females, which was lower than the diagnostic criteria. As WC has been shown to increase with aging [18], management of body shape is needed from young adulthood.
This study has several research limitations. First, it was designed as a cross-sectional survey, which cannot provide a causal inference or demonstrate a mechanistic connection between MO and lifestyle habits. Second, the study was conducted using a self-administered questionnaire, which may not accurately assess lifestyle habits. Third, regarding dietary habits, as we did not assess intake of nutrients in detail, caution should be exercised in interpretation. However, this is the first study to investigate the relationship between MO and lifestyle habits and the effects of MO on health among Japanese university students. The present results provide new evidence on MO management in university students.

Conclusion
The percentages of students with MO in 2019 were 13.4% for males with a decreasing trend, and 25.8% for females with an increasing trend. MO was negatively associated with the intake of five macronutrients, balanced diet intake, or exercise habits. Hypertension was significantly associated with BMI, WC, and MO in males, and with WC in females. These findings suggest that intervention for MO is needed in female university students, which could be achieved by adjusting their eating and exercise habits. Furthermore, as MO in young people may be a risk factor for hypertension, intervention from a young age is important.
Supporting information S1 Table. Questionnaire entitled survey on lifestyle habits. (DOCX)